1. Field of the Invention
The present invention relates to a frequency modulation corrector and, more particularly, to a circuit for correcting the modulation degree to frequency in for example a voltage-controlled oscillator (VCO).
2. Description of the Background Art
In general, a voltage-controlled oscillator, used in a wireless device of a conventional direct frequency modulation system, may be represented by an equivalent circuit including, as a resonant circuit, a fixed inductor, a first variable capacitor element for selecting the carrier frequency and a second variable capacitor element responsible for frequency modulation. The first and second variable capacitor elements are connected in parallel to each other. The voltage-controlled oscillator also includes an amplifier circuit for use in oscillating the resonant circuit. The voltage-controlled oscillator, formed by this resonant circuit, selects the carrier frequency by the direct current (DC) potential applied to the first variable capacitor element, while setting a modulation degree to frequency by the DC potential applied to the second variable capacitor element.
In the voltage-controlled oscillator, the capacitance value of the first variable capacitor element is fixed, during data transfer, as the capacitance value of the second variable capacitor element is changeable in response to a data pattern. It should be noted that the second variable capacitor element is controlled in such a manner that the voltage applied across the element is varied with time to cause changes in the capacitance value. This circuit structure, however, suffers from a deficiency that the modulation degree to frequency is increased with the increasing carrier frequency.
Let the frequency of the carrier wave, the inductance of the inductor, the capacitance value of the variable capacitor element determining the carrier frequency and the capacitance value of the other variable capacitor element responsible for frequency modulation be f, L, C and CMOD, respectively. Since the oscillation frequency f is given by f=½π{(Ci+CMOD)L)}1/2, the modulation degree to frequency (Δf)i for the carrier frequency fi may be expressed by an approximation:(Δf)i/fi=−(½)×(ΔCMOD)i/(Ci+(ΔCMOD)i)≈−(½)×(ΔCMOD)i/Ci.  (1)
It should be noted that the approximation of the expression (1) becomes possible by the fact that the capacitance value Ci of the variable capacitor element determining the carrier frequency f is usually larger by two orders of magnitude than the capacitance value (ΔCMOD)i of the other variable capacitor element responsible for frequency modulation. For the case of a carrier wave of different frequencies f1 and f2, the ratio of the modulation degrees to frequency (Δf)1 and (Δf)2 is given by(Δf)2/(Δf)1=(f2/f1)×(C1/C2)×[(ΔCMOD)2/(ΔCMOD)1]=(f2/f1)3×[(ΔCMOD)2/(ΔCMOD)1],   (2)by applying the approximation.
In the conventional circuit system, a preset DC potential is applied to a variable capacitor element, having the variable capacitance value ΔCMOD, the ratio of-the modulation degrees to frequency is proportionate to the third power of the ratio of the carrier frequencies, as indicated by the expression (2). For example, if a conventional direct frequency modulation circuit is used in the restricted radiation frequency band for industrial, scientific and medical equipment (f=2.4 to 2.5 GHz), the modulation degree to frequency for 2.5 GHz is larger than that for 2.4 GHz by a factor of (2.5/2.4)3=1.130. This carrier frequency dependency of the modulation degree to frequency may practically not be discounted, such that the modulation degree to frequency needs to be constant without dependency on the carrier frequency. For this reason, the variation of the variable capacitance which affords the frequency modulation must be controlled in dependence upon the carrier frequency defined by the expression (3) to maintain the relationship:(ΔCMOD)2=(f1/f2)3×(ΔCMOD)1.  (3)
Among more specified instances for suppressing the changes in the modulation degree to frequency, there is a voltage-controlled oscillator described in the Japanese Laid-Open Patent Publication No. 2001-196860. In this voltage-controlled oscillator, there is provided, in addition to the first variable capacitor element determining the carrier frequency and the second variable capacitor element responsible for the frequency modulation, a third variable capacitor element, connected in parallel with the first and second variable capacitor elements, and both the alternate current (AC) signal for frequency modulation and the DC signal determining the carrier frequency are applied to the third variable capacitor element to correct the aforementioned effect proportionate to the third power of the carrier frequency ratio by a ratio of the modulation degree to frequency.
Meanwhile, since the aforementioned Japanese Laid-Open patent Publication No. 2001-196860 uses, in addition to the first and second variable capacity elements used in the resonant circuit, the third variable capacitor element to compensate for the aforementioned effect, resulting in the resonant circuit being complicated in structure and bulky due to the additional constituent element.
Not only is the third variable capacitor element added in the resonant circuit, but also a resistor is introduced, as a result of provision of the third variable capacitor element, across the first and third variable capacitor elements. This resistor is used for decreasing the DC voltage applied to the third variable capacitor element. As aforesaid, this DC voltage determines the carrier frequency. Moreover, a fixed capacitor element has to be connected across the second and third variable capacitor elements. The purpose of providing this fixed capacitance is to prevent the DC voltage, determining the carrier frequency, from being applied to the second variable capacitor element.
In this technique, applied for suppressing the carrier frequency dependency of the modulation degree to frequency, the resistor, introduced as described above, and a resistance component of the third variable capacitor element, operate as a noise source of an oscillator, as a result of which noise characteristics, represented by for example the signal-to-noise (S/N) ratio, are deteriorated. This deterioration of the noise characteristics is not such as may be tolerated in the case of the oscillator implemented in the form of a semiconductor integrated circuit. Specifically, the oscillator, implemented as a semiconductor integrated circuit, is designed to satisfy the specifications of the noise characteristics with only a narrow allowance under such constraint as reduction of the occupied space or suppression of the power consumption and under such a condition in which there is provided neither the third variable capacitor element nor the resistor. In other words, this designing introducing the third variable capacitor element and the resistor would cause no room to be left for tolerating the rise in noise level.
If nevertheless the designing is changed to introduce the third variable capacitor element and the resistor and to meet the specifications for the noise characteristics in the oscillator or resonant circuit, the space occupied or the power consumption would be increased, so that the competitive power of the product is lost.